The present invention is an improvement on the optical system described in U.S. Pat. No. 4,391,494, issued July 5, 1983, to Ronald S. Hershel and assigned to General Signal Corporation. The system described in the aforementioned patent is a unit magnification achromatic anastigmatic optical projection system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures. The system is basically symmetrical, thus eliminating all aberrations of odd order such as coma, distortion and lateral color. All of the spherical surfaces are nearly concentric with the centers of curvature located close to the focal plane. Thus the resultant system is essentially independent of the index of refraction of the air in the lens making pressure compensation unnecessary. However, in order to attain sufficient working space for movement of the reticle and wafer, the object and image planes of this system are separated through the use of two symmetrical folding prisms. The cost of this gain in working space is the reduction of available field size to about 25% to 35% of the total potential field. In the past, this reduction in field size has not been critical since it has been possible to obtain both acceptable field size and the degree of resolution required for state-of-the-art circuits However, with increasing demands for higher resolution capabilities from such systems, applicant has recognized a need to modify the system so that even higher numerical apertures and higher resolution may be obtained while maintaining acceptable field size.